Electrochemical machining (ECM) channels an electrolyte in liquid form between a cathode tool and an anode workpiece to electrochemically erode or machine away material from the workpiece for obtaining a desired shape. The workpiece is typically mounted to a shuttle which fixedly mounts k in an ECM machine and provides a good electrical path for the high electrical currents channeled therethrough during ECM operation. The ECM machine typically also includes various electrical insulators and seals for confining flow of the electrolyte so that only the workpiece is machined and stray electrochemical machining of unintended material is prevented. Stray electrochemical machining, for example, may unintentionally damage portions of the workpiece itself rendering the workpiece unusable as a finished product.
An exemplary workpiece is a gas turbine engine stator vane having an airfoil and integral outer and inner shrouds. The shrouds, also known as platforms, provide flowpath boundaries for the motive fluid such as compressed air or combustion gases channeled over the airfoil during operating in a gas turbine engine. In order to electrochemically machine the workpiece into its final stator vane configuration, it is desirable to simultaneously electrochemically machine both the airfoil portion of the workpiece as well as at least one of the shroud portions of the workpiece with a common cathode tool.
One exemplary method and apparatus for electrochemically machining such a stator vane is disclosed in U.S. patent application Ser. No. 07/879,791, filed May 6, 1992, entitled "MultiCathode ECM Apparatus, Method, and Product Therefrom," by N. A. Bruns et al, assigned to the present assignee. In the Bruns et al ECM machine, the cathode tool is moved simultaneously towards both the airfoil and the shroud held in a workpiece shuttle for simultaneously electrochemically machining both portions in a single operation. However, the original workpiece typically is prefinished below the shroud or platform top surface and must be suitably protected from stray electrochemical machining thereof which would damage the shroud and render it unusable. Furthermore, the corners formed between the shroud top surface and its side surfaces are typically very sharp, 90.degree. corners which are required for assembly in a complete ring in the gas turbine engine. Stray electrochemical machining of these corners which causes them to be rounded off is undesirable and can also render the part unusable.